Designation F1844 − 97 (Reapproved 2016) Standard Practice for Measuring Sheet Resistance of Thin Film Conductors For Flat Panel Display Manufacturing Using a Noncontact Eddy Current Gage1 This standa[.]
Trang 1Designation: F1844−97 (Reapproved 2016)
Standard Practice for
Measuring Sheet Resistance of Thin Film Conductors For
Flat Panel Display Manufacturing Using a Noncontact Eddy
This standard is issued under the fixed designation F1844; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision A number in parentheses indicates the year of last reapproval A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1 Scope
1.1 This practice describes methods for measuring the sheet
electrical resistance of sputtered thin conductive films
depos-ited on large insulating substrates (glass or plastic), used in
making flat panel information displays
1.2 This practice is intended to be used with Test Methods
F673 This practice pertains to a “manual” measurement
procedure in which an operator positions the measuring head
on the test specimen and then personally activates the test
apparatus The resulting test data may be tabulated by the
operator, or, alternatively, sent to a computer-based data
logging system Both Methods I and II of Test MethodsF673
(paragraphs 3.1 through 3.3.3 of Test Methods F673) are
applicable to this practice
1.3 Sheet resistivity in the range 0.020 to 3000 Ω per square
(sheet conductance in the range 3 by 10–4 to 50 mhos per
square) may be measured by this practice The sheet resistance
is assumed to be uniform in the area being probed
N OTE 1—Typical manual test units, as described in this practice,
measure and report in the units “mhos per square”; this is the inverse of
“ohms per square.”
1.4 This practice is applicable to flat surfaces only
1.5 This practice is non-destructive It may be used on
production panels to help assure production uniformity
1.6 The values stated in SI units are to be regarded as
standard No other units of measurement are included in this
standard
1.7 This standard does not purport to address all of the
safety concerns, if any, associated with its use It is the
responsibility of the user of this standard to establish
appro-priate safety and health practices and determine the
applica-bility of regulatory limitations prior to use.
2 Referenced Documents
2.1 ASTM Standards:2
F673Test Methods for Measuring Resistivity of Semicon-ductor Slices or Sheet Resistance of SemiconSemicon-ductor Films with a Noncontact Eddy-Current Gage(Withdrawn 2003)3
3 Summary of Practice
3.1 This practice describes the preferred means of applying Test MethodsF673to measure the electrical sheet resistance of thin films on very large, flat, nonconducting substrates The substrate, oriented with the conducting thin film up, is placed between the transducers of the eddy current sensor assembly at the point of interest The test arrangement is illustrated inFig
1 3.2 A typical conductance apparatus is described in detail in
a paper by Miller, Robinson, and Wiley.4 This paper also discusses skin-depth as a function of thickness and resistivity 3.3 A typical apparatus operates as follows: when a speci-men is inserted into the fixed gap between the two parallel sensing elements, or transducers, in a special oscillator circuit, eddy currents are induced in the specimen by the alternating field between the transducers The current needed to maintain constant voltage in the oscillator is determined internally; this current is a function of the specimen conductance
3.4 Further details are given in Test MethodsF673, para-graphs 3.1 through 3.3.3
3.5 This practice includes calibration procedures for using NIST Silicon Standard Reference Material5 to ensure proper operation before testing panels
1 This practice is under the jurisdiction of ASTM Committee F01 on Electronics
and is the direct responsibility of Subcommittee F01.17 on Sputter Metallization.
Current edition approved May 1, 2016 Published May 2016 Originally
approved in 1997 Last previous edition approved in 2008 as F1844 – 97(2008).
DOI: 10.1520/F1844-97R16.
2 For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
3 The last approved version of this historical standard is referenced on www.astm.org.
4 Miller, G.L., Robinson, D A H., and Wiley, J D., “Contactless Measurement
of Semiconductor Conductivity by Radio Frequency-Free-Carrier Power
Absorption,” Review of Scientific Instruments , Vol 47, No 7, July 1976.
5 Available from NIST, 100 Bureau Dr., Stop 3460, Gaithersburg, MD 20899.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959 United States
Trang 24 Significance and Use
4.1 Resistivity is a primary quantity for characterization and
specification of coated glass plates used for flat panel displays
Sheet resistance is also a primary quantity for characterization,
specification, and monitoring of thin film fabrication processes
4.2 This practice requires no specimen preparation
4.3 The eddy current method is non-destructive to the thin
film being measured Special geometrical correction factors,
needed for some four-point probe electrical resistivity
measurements, are not required to derive the true sheet
resistance so long as the transducers have a continuous layer of
conductive thin film between them
4.4 Test MethodsF673 refers to a testing arrangement in
which the transducers and specimen (a semiconductor grade
silicon wafer) are rigidly positioned Similar apparatus is
commercially available for testing large glass or plastic
substrates, not envisioned in the scope of Test MethodsF673
A hand held probe can also be used, depending on throat depth
required
4.5 For use as a referee method, the probe and measuring
apparatus must first be checked and qualified before use by the
procedures of Test Methods F673 (9.1.1 through 9.1.3 and
9.1.4.2 through 9.1.4.5), then this practice is used
4.6 For use as a routine quality assurance method, this
practice may be employed with periodic qualifications of probe
and measuring apparatus by the procedures of Test Methods
F673 (9.1.1 through 9.1.3 and 9.1.4.2 through 9.1.4.5) The
parties to the test must agree upon adequate qualification
intervals for the test apparatus
5.1.1 Different transducer designs may be required to cover the full range of sheet resistance values
N OTE 2—Three transducers will generally cover the ranges of interest For convenience these are denoted “High” (15 to 3000 Ω per square),
“Low” (0.2 to 15 Ω per square) and “Extra Low” (0.035 to 0.2 Ω per square).
N OTE 3—The usual “High” range transducer diameter is approximately 12.7 to 15.2 mm The “Low” and “Extra Low” diameter is approximately 10.1 to 12.7 mm A very large transducer, 63.5-mm diameter, may be used for all ranges for thicker than normal substrates (up to approximately 2.54 mm) and for calibration and measurement ease.
5.2 Electrical Measuring Apparatus —The electrical
appa-ratus must meet the requirements of Test Methods F673, paragraphs 6.1 through 6.4
5.3 Specimen Support— The flat panel to be tested must be
supported firmly to ensure that the thin film is parallel with the transducer surfaces
5.4 Reagents and Materials in accordance with Test
Meth-odsF673, Section 7
6 Test Specimen
6.1 The test article shall be a display substrate that has been sputter coated with the conductive thin film of interest or ion implanted and annealed, or made conductive by another process
6.2 The conductive film must be thick enough that it is continuous Generally this requires that the film be at least 15-nm thick
6.3 The area to be tested shall be free of contamination and mechanical damage, but shall not be cleaned or otherwise prepared
6.4 Note that a sputtered film may also coat the edge of the glass and can coat the back side of the substrate (“overspray”) All overspray, for example, coating on back of glass, must be removed before measurement
6.4.1 Any remaining overspray will be included in the measurement, lowering the measured film resistivity
6.4.2 Scribing the substrate near the edge using a glass scribe is not a reliable remedy
6.4.3 Use a simple 2-point probe ohmmeter to verify that the back side of glass or plastic substrate is insulating
7 Interferences
7.1 Caution must be taken that the transducer gap is fixed, in accordance with the recommendations of the equipment sup-plier This may be ensured by firmly tightening the gap adjustment screws after checking the spacing with gages Use caution, too, that the electrostatic covers (see Miller, et al.4) are not damaged by the panel under test The electrostatic cover should be located approximately 0.02 mm below the support surface
7.2 Radial resistivity variations or other resistivity
nonuni-N OTE 1—This figure is partially copied from Fig 1 of Test Methods
F673
FIG 1 Schematic Diagram of Eddy-Current Sensor Assembly
Trang 3those of four-probe measurements depending on film
proper-ties and the four-probe spacing used (see Test Methods F673
paragraph 5.1)
7.3 Spurious currents can be introduced in the test
equip-ment when it is located near high-frequency generators If the
equipment is located near such sources, adequate shielding
must be provided Power line filtering may also be required
(Note the precautions in Test MethodsF673paragraph 5.4)
7.4 Soda Lime Glass Substrates—Special care may be
required in measuring the sheet resistance of sputtered thin
films on soda lime glass substrates The surface of this glass
can be somewhat electrically conductive (on the order of 1 ×
106Ωper square) when the ambient relative humidity is 90 %
or higher
7.4.1 The glass conductivity degradation may interfere with
the sheet resistance measurement when specimen sheet
resis-tivity is 1000 Ω/square or higher
7.4.2 Ensure that films > 1000 Ω/square sheet resistance
deposited on soda lime glass are conditioned at less than 50 %
humidity for at least 48 hs prior to measurement, and that the
measurement is performed at an ambient relative humidity of
less than 50 %
N OTE 4—At relative humidity < 50 % of the surface resistance of soda
lime glass is on the order of 1 by 10 12 Ω/square.
8 Procedure for Fabrication and Use of Sheet Resistance
Reference Specimens
8.1 It is useful to maintain sheet resistance reference
speci-mens for use in verifying the proper performance of the
measuring apparatus (seeFig 2)
8.1.1 Rectangular sheets of etched glass nominally 50 by 75
mm are suitable substrates The roughness of the etched surface greatly improves abrasion resistance
8.1.2 The reference film, applied to the substrate, may be a nominally 40-nm thick sputtered tin-oxide coating, doped with nominally 5 wt % antimony or fluorine This material dem-onstrates good chemical stability and abrasion resistance, and sheet resistance on the order of 1500 Ω/square
8.1.2.1 Tin oxide is a photo conductor with very long carrier lifetimes Thus the lighting conditions must be controlled to prevent exposure to direct light, or the film must be recali-brated before each use
8.1.3 A double layer of nominally 100-nm sputtered indium-tin oxide at 90/10 composition ratio covered with 40-nm doped tin oxide (paragraph 8.1.2) for abrasion resis-tance forms a satisfactory reference film in the 25-Ω/square sheet resistance range The photo conductive effect is negligible, but films may exhibit long term resistivity drift Periodic recalibration is required
8.1.4 After applying the reference film, highly conductive bus bars, nominally 12.5 mm wide, are deposited over the film along two opposite“ short” edges of the substrate, as illustrated
inFig 2 The free conducting area of film is thus a nominally
50 by 50 mm square
8.1.4.1 A sputtered chromium adhesion layer, nominally 100-nm thick, upon which is sputtered a thick copper conduc-tive layer nominally 1000 nm with a sheet resistance of 50 milliOhms per square or less is a satisfactory bus electrode for reference films of 20 Ω per square or greater Reference films less than 20 Ω per square should have a copper wire soldered
FIG 2 Calibration Sample
Trang 4to the lengths of the bus electrodes, or should have the
thickness of the copper film electrodes increased
proportion-ately
8.1.4.2 The sheet resistance of the reference film may be
calibrated using a 2-point or 4-point method, using the bus bars
as contact lines The measured V/I ratio is the sheet resistance
for the square reference sample No correction factors are
required
8.1.5 The conditions and precautions prescribed in 7.2 –
7.4.2pertain to sheet resistance reference specimens
8.1.6 The probe and associated measuring apparatus are
checked by applying the measuring procedure (Section10), to
the reference film with the probe near the center of the
reference film
9 Calibrations
9.1 If using a NIST Standard Reference Material (SRM) (or
specimen as prepared under Section8), refer to Test Methods
F673paragraphs 9.1.1 through 9.1.3
9.2 Position the center plane of the thickness of the S.R.M
between the eddy current transducers in the same plane as that
occupied by the conductive film of the flat panel during actual
sheet resistance measurements This is illustrated inFig 3
9.3 Continue with Test Methods F673 paragraphs 9.1.4.2
through 9.1.4.5
10 Measurement Procedure
10.1 Position the flat panel so the surface of the thin film is
facing “up” and is parallel with the flat surfaces of the
transducers, with the thin film plane lined up where the
calibration SRM center was This is illustrated inFig 3
10.2 Measurement Procedure:
10.2.1 Because of differences in manufacturer’s
recom-mended procedures for the various flat panel display measuring
instruments, refer to the operator’s manual for explicit
mea-surement instructions for the equipment employed The steps
following are general statements, intended to outline the
procedure
10.2.2 Manually zero the instrument or record the
resistivity/conductivity meter reading If reading the meter
without zeroing the instrument, this is the “offset.”
10.2.3 Place flat panel display on base with the conductive
thin film side “up” (in the same position as shown inFig 3) so
that the region to be measured is roughly centered in the transducers’ “footprint” area
10.3 If previously zeroed, the reading taken after the flat panel display is placed between the transducer heads is the sheet conductance in mhos per square
10.4 If not previously zeroed, subtract the “offset” from the second reading The result is the sheet conductance in mhos per square
11 Report
11.1 For a referee test the report shall contain the following: 11.1.1 Operator name, date, description of test equipment, 11.1.2 A description of the specimen, including:
11.1.2.1 Type of film, 11.1.2.2 Specimen identification, and 11.1.2.3 Brief description of visual appearance and physical condition,
11.1.3 Dimensions and data, including:
11.1.3.1 Length and width of specimen, 11.1.3.2 Description of eddy current instrument including transducer diameter and spacing between the two transducers, and
11.1.3.3 Measurement system validation data obtained from testing one or more reference specimens (Section9) or NIST Silicon Standard Reference Materials When using SRM wa-fers they should be supported so that the center of the wafer thickness is in plane with the center of the flat panel thin film 11.1.4 Ambient temperature and humidity,
11.1.5 Measurement locations, 11.1.6 Measured individual values of sheet resistance, and 11.1.7 Computed average sheet resistance, and standard deviation
11.2 For a routine test, only such items as are deemed significant by the parties to the test need be reported
11.3 Film uniformity— A recommended method of
describ-ing film uniformity for the rectangular substrates is to measure
the sheet resistance Rsin five or more locations, typically near
the four corners and at the center The uniformity measure, U,
is computed from the following equation:
U 5 100~Rsmax 2 Rsmin!/~Rsmax1Rsmin!%, (1)
where Rsmax and Rsmin are the maximum and minimum respectively of the five measured sheet resistance values
Trang 512 Precision and Bias
12.1 Standard deviation, s, from measured sheet resistance,
Rs, is to be determined by pilot study and subsequent
inter-laboratory test
13 Keywords
13.1 conductance; electrical conductance; electrical
resis-tance; electrical sheet conducresis-tance; electrical sheet resisresis-tance;
flat panel displays; noncontact eddy current; resistance; sput-tered thin films; thin conductive films on glass; thin film
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